LESSON 1: QUALITIES OF DIGITAL IMAGES
- Bit Depth: 8-bit Vs. 16-bit
- Sharpness vs. Focus
- Dynamic Range
- Image Noise
- Image File Formats, Resolution & Size
- The On-Screen Image Resolution Myth Busted
Image File Formats
One of the main criteria that distinguishes different file formats is image compression. Compressed files’ size is significantly smaller and allows us to send or upload them on the web, or simply have a higher number of images stored on a memory card or computer.
The downside of image compression is that it can be lossy. While lossless compression preserves image information (colors, sharpness & detail) leaving the file size slightly larger, lossy compression, on the other hand, reduces the file size much further, and discards a lot of image data, so the quality of the final image may suffer greatly.
Lossy data compression involves discarding some of the media’s data so that it becomes simplified enough to be stored within the desired disk space (Wikipidia on Compression Artifacts).
Raw format captures most of the light and color information, offers greater control in post-production and there’s no image compression involved. You don’t have to worry too much about your White Balance settings and over- or underexposing a little bit as you shoot. You can even assign color profiles (sRGB, Adobe RGB or ProPhoto RGB) to your images in your Raw converter (Adobe Lightroom, Camera Raw, etc.). Although, I usually encourage my students to choose correct camera settings before the images are taken – that will always save you post-production time. But at the end of the day you have better chances to end up with greater image quality and not ruin your photos if you shoot in Raw.
Raw file formats are extremely popular in digital photography these days, however, different camera manufacturers created different raw formats for their cameras, which means that not every raw file can be read by a variety of software applications. As a result, when you are using these proprietary raw files as a long-term archival solution you are risking not being able to open and work with them in the future, if the original raw file format you have saved them in becomes redundant.
The solution to this is Digital Negative (DNG), a publicly available archival format for the raw files generated by digital cameras. Hundreds of software manufacturers such as Apple and Google have developed support for DNG. Some respected camera manufacturers such as Leica, Casio, Ricoh, Samsung, and Pentax even introduced cameras that provide direct DNG support.
I always convert my images to DNG (Digital Negative) format as I download them from my camera. DNG format will not become redundant like the proprietary camera formats may.
PSD & PSB (Photoshop document) is a universal format and therefore a logical choice when saving and archiving your master files (with unmerged layers). It’s the most efficient format to save files without degrading the quality of the image in any way.
PSB accommodates files greater than 2GB in size (video editing, large print designs, etc.)
TIFF is used widely in the publishing industry on both Mac and Windows platforms. It supports Photoshop layers and layer styles, but is most often used for delivering print images to a client, since it doesn’t compress file information like JPEG.
JPEG or JPG is a lossy compression format, which means that you lose quality every time you open and save a JPEG file. It applies additional compression to the image discarding more color/pixel information each time you re-save it.
JPEG stands for “Joint Photographic Expert Group” and was specifically developed for storing photographic images.
I only save my final retouched photos that I’m going to share on the web in JPEG. I also often deliver images to my private and commercial clients in high resolution JPEGs if other formats are not requested specifically.
- Shoot in raw format
- Convert your images to DNG when downloading from your camera
- Save your masterfiles in Photoshop documents (PSD)
- Save copies of your retouched images in low resolution JPEG for web usage (read on image resolution below)
- Always ask your client what file format they prefer for you to deliver their final images in (JPEGs or TIFFs). Note: I never give away my PSD (masterfiles).
Image Resolution is a term that describes how much detail an image holds and refers to its sharpness and clarity. Higher resolution means more image detail in printed images.
In the case of dot-matrix and laser printers, the resolution indicates the number of dots per inch.
dpi = dots per inch (for print images) For graphics monitors, the screen resolution signifies the number of dots (pixels) on the entire screen.
ppi = pixels per inch (for on-screen digital images)
RESOLUTION MYTH BUSTED
It’s been commonly misunderstood (I am not an exception), that the Resolution in the Image Size menu in Photoshop represents both the print resolution of the document and the screen resolution of the file. However, the Resolution number only dictates the amount of dots/pixels per inch of your image when it’s printed (that’s actually why it is located in the Document Size section of the Image Size menu – marked pinked in the illustration below). It’s the pixel dimensions that really count for an image in any screen based media (e.g. a website, your desktop), its resolution does not matter.
Read more about it in our The On-Screen Image Resolution Myth Busted article.
Yes, that’s right, resolution has nothing to do with your on-screen images, and 72ppi or 96ppi are nothing but misleading numbers from the past. You could use a 300ppi or 1 ppi (two ppi) resolution setting and see exactly the same result on screen. In fact, we could use it to our advantage and share all our images with 1dpi resolution online, to prevent stealing them for printing.
Still not convinced? Try saving the same picture 600px wide with resolution 300 ppi, and two more copies of it also 600px wide and resolutions 1 ppi and 72 ppi. You will see the same quality (when viewed on screen) and the file size will be the same in all three versions.
However, it is still important to save images that are destined to be printed in high resolution!
- For all your image file that you will share on the web, feel free to set 1 ppi for Resolution – it will not decrease the quality of your on-screen images, moreover it will prevent from unauthorized printing.
- Save your masterfiles (.PSD) and images destined for print in high resolution. I personally choose 300 ppi, but if your printer’s resolution is even higher, set the resolution of your image to match it.
IMAGE SIZE & CANVAS
Image Size dialog box is divided up into two main sections – the Pixel Dimensions section on top, and the Document Size section below it (see illustration below).
The Pixel Dimensions section describes the file size of the image, its width and height in pixels for on-screen images (those that you share on the web).
The Document Size section tells us how large or small our image is going to print, based on the image resolution.
You can change what units you’d like to use by default for the size of your PSD documents via Preferences > Units & Rulers.
Scale Styles option has to do with Layer Styles and whether they are affected (checked) or not (unchecked) when the image is being resized or resampled.
Default setting – checked.
Constrain Proportions links the width and height of the image together, so that if you make a change to one of the dimensions of the image, its proportions will remain the same.
Default setting – checked.
Resampling Image is changing the amount of image data as you change either the pixel dimensions or the resolution of an image.
Default setting – checked, bicubic (best for smooth gradients).
When you downsize an image (decrease the number of pixels), information is deleted from it. When you enlarge an image (increase the number of pixels), new pixels are being added. Interpolation is the process of filling in the unknown data between two known values, in other words creating new pixels and guessing their values by referencing the surrounding pixels – Bicubic interpolation.
Bilinear interpolation calculates new pixels by reading the horizontal and vertical neighboring pixels (it is not as accurate as Bicubic).
You can choose which interpolation method to use in the Resample Image dropdown menu at the bottom of the Image Size dialog box (see figure above). To specify a default interpolation method you would like Photoshop to use whenever it resamples image data go to Preferences > General > Image Interpolation.
I’ve already mentioned that Bicubic is more accurate way of resampling an image, but which Bicubic type to choose? I personally have been using Bicubic Automatic since I upgraded to CS6 (that’s when it first appeared in the Resampling dialog box), and even though it sharpens more heavily than any of other types, I still apply additional sharpening to specific parts of an image anyway, so I don’t mind the Automatic sharpening. If you like softer images, test all there types for down-sizing: Bicubic (best for smooth gradients), Bicubic Sharper (best for reduction) and Bicubic Automatic and see what suits your taste better.
PRO TIP: ENLARGING IMAGES
When enlarging an image, make sure all three options are checked and Resample Image method is set to Bicubic Smoother. Enlarge your image increasing its percentage (found in Pixel Dimensions units – see illustration below) in a few rounds, not more than 120% at a time.
Click on the Actions icon to download a few Photoshop Actions for quality enlarging and down-sizing for web usage.
Unzip downloaded file and install RA – Handling & Re-sizing Files.atn into your Actions panel.
Included Actions are used for:
RA – Increase print size 120% – for quality enlargement: You can run this Action a few times increasing your image size 120% each time until you get to the desired size. Can be used for both high and low resolution images.
RA – Down-size VERT 600px LOW RES: Run one time to down-size any large vertical (portrait format) image to 600px wide and 1ppi – I use this Action to create a copy of a retouched image to share on the web.
RA – Down-size HOR 900px LOW RES: Run one time to down-size any large horizontal (landscape format) image to 900px wide and 1ppi. Used same as the Action above.
Enjoy saving time!
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